The maintenance of cellular water balance is vital to all organisms. Every cell tested has some capacity to regulate the amount of intracellular water when exposed to an osmotic stress. This process of cell volume regulation involves the control of intracellular, osmotically active, solute levels. Under conditions of hypoosmotic stress this solute is extruded from the cell together with osmotically obligated water and cell volume is maintained. The solute concentration is increased during hyperosmotic stress, water is drawn into the cell and osmotic shrinkage prevented. While this phenomenon is common to all cells, the cells of osomoconforming euryhaline marine invertebrates do it best and we have been using these species as model systems to study the process. The source of osmotic solute in all cells seems now to be some combination of inorganic ions and free amino acids or other small organic molecules. The exact regulatory mechanisms are unknown, but our previous work has indicated that separate membrane permeability control systems are utilized for each solute type, but these systems operate in a coordinated manner. The basis of this coordination is also unknown. Based on our previous work I have proposed a group of experiments designed to describe the nature of the inorganic solute control system (we already have a lot of information about the amino acid system) and to test the possibilities that the interaction of these membrane systems is mediated by external Ca++ concentrations by means of cytoskeletal actin. Finally, we will test the possibility that the permeability systems are hormonally controlled and that this control is by membrane protein phosphorylation mediated by cyclic AMP. I have chosen 3 cell types for study, based on our previous work, red blood cells from the clam Noetia ponderosa; red coelomocytes from the worm Glycera dibranchiata and walking leg muscle from the horeshoe crab Limulus polyphemus. The results of this research will provide specific information about the mechanisms of cell volume regulation and membrane permeability control.